Functionalized SiO2 nanoparticle-mediated release and transport of Cd through soil porous media: implications for their application in soil remediation

Silica-based nanomaterials have shown promise for remediating heavy metal-contaminated soils. However, their remediation stability may be compromised by natural and anthropogenic activities such as rainfall and irrigation. This work systematically investigates the leaching behavior of Cd from soils amended with unmodified SiO 2 and thiol-functionalized SiO 2 (SH-SiO 2 ) nanoparticles, along with nanoparticle-mediated Cd release and transport through soil porous media. The results demonstrated that the leaching concentrations of Cd 2+ from the soil remediated by SH-SiO 2 were the least and followed the order of SH-SiO 2  < SiO 2 ≈ control (untreated Cd-contaminated soil) under simulated acid rain conditions (pH 5.6). This confirms that thiol-functionalized silica effectively immobilizes Cd due to its strong chelating ability. However, co-transport experiments revealed that both SiO 2 and SH-SiO 2 nanoparticles could act as carriers that enhanced Cd 2+ transport, governed by their Cd-binding capacity and intrinsic mobility over a pH range of 5.0–8.0, with greater facilitation at higher pH levels. Notably, SH-SiO 2 exhibited a much stronger facilitation effect, attributable to its higher affinity for Cd 2+ , greater mobility in porous media, and resistance to desorption of SH-SiO 2 -bound Cd 2+ . At a nanoparticle concentration of 50 mg/L, the maximum breakthrough of Cd 2+ reached 10.1% for SiO 2 and 75.2% for SH-SiO 2 . While SH-SiO 2 demonstrates excellent Cd immobilization capacity and acid-rain resistance, any disturbance that remobilizes these nanoparticles may facilitate the mobility of Cd 2+ , potentially expanding the scope of heavy metal contamination. Therefore, when applying remediation agents for soil rehabilitation, it is crucial to consider their long-term effectiveness and stability.